Wire broadcasting system



Feb. 18, 1936.

P. P. ECKERSLEY ET AL 31 9 WIRE BROADCASTING- SYSTEM Filed July 26, 1954Fig. 2.

28 7 PETER Pzuog ron EcKERsLEY 6a RUPERT EVAN HowARD QARPENTER BY g @ATTORNEYS Patented Feb. 18, 1936 PATENT orrics WIRE BROADCASTING SYSTEMPeter Pendleton Eckersley, Chelsea, London, and

Rupert Evan Howard Carpenter, London, England Application July 26, 1934,Serial No. 737,120 In Great Britain August 1, 1933 9 Claims.

This invention relates to systems for distributing music, speech andentertainment or other signals, in general from a central station to anumber of receivers and including systems of the kind now usually knownas re-diifusion systems. The invention is particularly concerned withmulti-programme systems by means of which two or more programmes aremade available by impressing modulated high frequency currents upon asuitable conducting network used for distributtion purposes and nowoften known as wirebroadcasting systems.

It is possible to distribute several programmes simultaneously in thisway by employing 'a number of different carrier frequencies for carryingthe different programmes. It is desirable because attenuation of thesignals increases with frequency, to make the difference in frequencybetween the different carrier frequencies employed as small as ispossible consistent with the attainment of good quality of reproduction.

The technique of high frequency diffusion over wires differs from thatof radio broadcasting, because the output terminals of each transmittingapparatus must be directly physically connected, while in broadcastingthe transmitting aerials are usually neither directly connected togethernor closely coupled. When the output terminals of two re-diifusiontransmitters are connected in parallel, the modulated high frequencyvoltages of one transmitter will be conveyed to the anodes of the outputtubes of the other transmitter and vice versa. If the direct current toalternating current efiiciency of the output tubes is reasonably high,it will follow that the signals from one transmitter will modulate thecarrier wave of the other transmitter since some rectification will takeplace in the output tubes when their anode potentials are raised andlowered 'by a certain amount. One of the purposes of the presentinvention is to make it possible to avoid this inter-modulationphenomenon in high frequency wire-broadcasting systems. 7

Such wire-broadcasting systems, however, have other peculiar problems oftheir own. Thus, it is foundthatthe impedance of the distributingnetwork isvery low compared with the impedance of the output tubes ofthe transmitters. This means, in effect, that a step-down transformerhas to be introduced between the tube anodes and the load. Moreover, asalready indicated, it is desirable in order to minimize the attenuation,to use carrier frequencies which at their lowest may be nearly audible.at a frequency, say of 10 kilocyclesper second,

If these are modulated the highest frequency of modulation may be asmuch as 50 per cent. of the carrier frequency. It follows from this thatthe high frequency power transformer used must have a sensibly fiatfrequency characteristic over a wide band of frequencies and the use ofordinary single frequency tuning arrangements, such as are commonlyemployed in wireless apparatus, would render it impossible to transmitthe required sidebands with substantially the same intensity. On theother hand, the secondary windings of the output transformers of all thetransmitters have to be connected to the same load, and unlessprecautions are taken this would result in the severe inter-modulationmentioned above.

Yet again, it is necessary to take steps to ensure that all, or nearlyall, of the power output from one transmitter shall pass into the loadand shall not be wasted in other circuits. If, however, for bestmatching results the impedance of the distributing network is equal tothe impedanceof the aperiodic power transformers, then each transmitterwill have a large part of its power wasted in every other transmitter.

There is yet another problem tobe solved which is concerned with thegeneration of harmonics, which, if transmitted, may either beat togetherin the receivers or produce direct interference on other channels.Harmonics may be generated particularly in the modulator and because ofthe necessity of using amplifiers giving a substantially uniformperformance over a wide band of frequencies, the harmonics may bestrongly amplified unless precautions are taken to suppress them.Furthermore, if the amplifying tubes and the output tubes are arrangedto have a reasonably high eificiency, they may themselves generateharmonics. Such generation of harmonics may, however, be minimized byoperating the tubes in phase opposition.

Thus, the provision of a multi-programme high frequency system workingon low carrier frequencies presents at least three distinct problems,comprising the prevention of inter-modulation, secondly the preventionof absorption of power from any transmitter by the output circuits ofthe other transmitter or transmitters, and thirdly the prevention of thetransmission of harmonics.

It is possible to solve all three of these problems, and to that endaccording to the present invention, a multi-programme high frequencywire broadcasting system includes an amplifier designed toamplify aselected band of frequencies substantially uniformly, astep-down'transformer or other network designed to cause the impedanceof the output stage of the amplifier to match the impedance of adistributing network, and a band-pass filter interposed between theamplifier and the distributing network. The band-pass filter may includea pair of series tuned circuits connected in series and having valuessuch that the point of comiection between them is a nodal point ofpotential, and a low-pass filter may be interposed between the outputstage of the amplifier and the distributing network, the low-pass filterbeing designed to prevent the transmission to the distributing networkof harmonic frequencies generated in the amplifier. Practicalconsiderations make it most convenient for the band-pass filteringdevice to be introduced between the secondary winding of the outputtransformer and the distributing network, but it is possible tointroduce it between the output tube of the transmitter and the outputtransformer. The band-pass filtering device in question may include apair of series tuned circuits having components of such magnitudes thatthe point of junction between them is at a minimum potential to ground,for a chosen frequency, generally the carrier frequency, and then a legof the filter may be connected between the junction point and groundwithout disturbing the distribution of energy at the carrier wavefrequency. In addition, rejectors may be interposed in the circuitsbetween the amplifiers and the distributing network. In order to preventabsorption of current from other transmitters, these rejectors.

must be connected on the distributing network side of any circuits orcircuit element capable of shunting such currents away from the network.The provision of the rejector circuits causes the frequency of any onetransmitter to find considerable difficulty in reaching the anode of theoutput tube of another transmitter connected to the network.

Yet again, in order to avoid interference between two transmittershaving their outputs directly connected together and feeding adistributing network by a single conductor of appreciable impedance tothe frequencies in question, the output from each separate transmittermay be taken to the network through a separate cable, preferably asheathed cable, the cores of which form the conductors from the outputof the transmitter to the different conductors of the network, while theouter conductor of the sheathed cable forms the grounding point from theoutput of the transmitter.

In order that the invention may be more clearly understood and readilycarried into effect, the output end of a high frequency re-diifusionsystem designed in accordance with the invention will now be describedby way of example in connection with the accompanying drawing, in which:

Figure 1 is a diagram showing the connections between the output ends ofthree separate transmitters and a mains network, and

Figure 2 shows an alternative form of rejector circuit which may beused.

Three transmitters each generate a selected carrier frequency and haveoutput stages A, B, C in the form of paraphase-connected amplifiers ofthe type described in patent application Serial No. 455,326, filed May24th, 1930, of R. E. H. Carpenter. These output stages include tubes Iand 2, the grid 3 of tube I being coupled with a preceding modulatedamplifying stage through a band-pass filter not shown in the figure, andthe grid of the tube 2 receiving an input in opposed phase from atapping in the resistance connected directly between the anodes of thetwo tubes 2, 3. The output from the tubes I and 2 is passed through amulti-section low-pass filter 4 including series-connected inductancesand shuntconnected condensers, the purpose of the filter being toprevent the transmission to the coupling transformer 5 of harmonicsgenerated in the preceding stages of the respective transmitters.Harmonics of one of the carrier frequencies if so transmitted, mightconceivably be sufliciently strong to cause interference with thesignals from another transmitter having a fundamental carrier frequencyequal to a given harmonic. Again, such harmonics may be picked up byradio receivers near the transmitters, or again by radio receivers usingthe lighting or power network D as an antenna system.

The transformer 5 has a step-down ratio and is designed to match thehigh inpedance of the tubes I and 2 to the comparatively low impedanceof the mains network D. The material use for the core 6 of thetransformer 5 must be such that it will not have a detrimental effectupon the transmission of high frequency currents.

In order to ensure the maximum transmission of power into the mainsnetwork D, it is necessary to reduce as far as possible, the: effect ofany reactive impedance which may be due, for example, to leakageinductance of the secondary winding of the transformer 5 and thereactance of the mains network D. The reactance of the mains network isusually inductive, and while a condenser connected in series wouldcancel both the inductive reactances due to the transformer and mainsnetwork, it would be fully effective only at one frequency. Thesecondary winding of the transformer 5 is therefore shown connected to aband-pass filter including series inductances I and 8 and seriescondensers 9 and Ill tuned to the carrier frequency, a condenser I Ibeing connected in shunt to ground from the connection common to thecondensers 9 and I5. It will be seen that as the secondary winding ofthe transformer 5 is in series with the inductance I and the condenser9, the leakage inductance due to the secondary winding forms part of theseries circuit of the band-pass filter. Again, the inductive reactancedue to the mains network is in series with the inductance 8 andcondenser I0, and by thus making these undesirable reactances part ofthe bandpass filter circuit, they are effectively disposed of.

The differing impedances at the sending and receiving ends of theband-pass filter may be accounted for by selecting condensers 9 and IIIof unequal values in order to ensure that the point of connectionbetween them is as nearly at a node in potential as possible. Theband-pass filter, when so adjusted, serves to transmit currents offrequencies equal to that of the carrier wave of the particulartransmitter with one or both sets of side bands added. However, whilethe bandpass filter does not transmit to any appreciable degree outsidethe range of carrier frequency with one or both sets of side bandfrequencies, it does, in fact, tend to behave differently at differentfrequencies within that range. The response curve representing outputplotted against frequency exhibits a cusp representing a reduction inresponse at a frequency which occurs mid-way in the band of frequenciestransmitted. At such a frequency, the sending and impedance of thefilter rises. The transformer 5, in conjunction with the preceding tubecircuits, may be therefore designed to give bad regulation, that is tosay, high internal impedance, and then the voltage of the outputtransformer 5 will tend to rise at the frequencies at which the cuspmentioned above occurs, so that the cusp in the curve will be lesspronounced. In order to emphasize the bad regulation, the transformationratio of the transformer 5 may be chosen to be somewhat less than thatwhich gives optimum matching of impedance. This introduces addedasymmetry in the bandpass filter, and it is then further necessary thatthe condensers 9 and II) should be unequal in value.

The band-pass filter assists in preventing the transmission of harmonicsand also assists in preventing inter-modulation between the respectivetransmitters A, B and C, because as it is tuned to pass frequenciesbetween the respective carrier wave frequencies and the highestside-band frequencies, it presents a high impedance to frequenciesoutside this band, and therefore highly attenuates such frequencies. Bymultiplying the separate sections of the filter, it is possible stillfurther to attenuate the frequencies it is desired shall not betransmitted, but attenuation of the carrier frequency then becomesappreciable, due to resistance.

If, in order to ensure efliciency, the image impedance of the band-passfilter of the transmitter A is equal to the terminating or loadimpedance, the impedance of the output end of the filter is so small atdifferent frequencies as to involve absorption of power from the othertransmitters B and C feeding contiguous frequencies into the mainsnetwork D.

It will be seen that all the transmitters A, B and C are connected to acommon mains network D, the currents from a given transmitter havingalternative paths either into the mains network D or through theband-pass filters of the other transmitters. If power from onetransmitter does pass into the circuit of another transmitter, not onlydoes wastage of power occur, but the anode voltage of the tubes of onetransmitter may be varied in accordance with the combined modulation ofthe other transmitters connected to the network D. inter-modulation maycause interference in respect of each subscribers receiving andreproducing equipment, and in order to prevent both intermodulation andwastage of power, rejector circuits l2-l3, I4l5, and I6--l'l areinserted in the feeders I8 from the transmitters to the network D. Therejector circuits have a high impedance at the frequencies it is desiredto reject. Thus, the filters l2 and I3 are designed to pass intothenetwork D the carrier and side-band frequencies from transmitter A, butthe filter I 2 will present a high impedance to the carrier andside-band frequencies from transmitter B, while the filter l3 willpresenta high impedance to the carrier and side-band frequencies fromtransmitter C. Similarly, the filters l4 and I5 will pass into thenetwork D the carrier and side-band frequencies from transmitter B,while presenting a high impedance respectively to the carrier andside-band frequencies from the transmitters A and C. Again, the filtersl6 and I! will pass into the network D the carrier and sidebandfrequencies from transmitter C while presenting high impedancesrespectively to the carrier and side-band frequencies from transmittersA and B. It will be understood that as the rejector circuits look likeinductances or capacities to the frequencies they are designed topass,the series components 1, 8, 9, and IU of' the band-pass behaveaperiodically.

filter must be selected of such magnitudes as to counter-balance theleft-over reactance of the 'rejector circuits.

As already indicated, the feeders Hi from the three transmitters A, B,and C to the common network D are preferably separated for eachtransmitter and only connected at the network load. In other words, itis preferable that there should not be any impedance common to all thetransmitters except the impedance of the network D itself, which isinevitable from the nature of' the problem. It does not matter if theseparate leads are self-inductive, because their re actance iscounter-balanced by the series condensers in the band-pass circuits overthe band of frequencies passed, but they should be free from mutualinductance. The feeders l8 are low resistance cables each having cores19 which form the conducting leads to the network, while the outersheath 20 of the cable forms'the ground point from the output of thetransmitter. The separate conductors 2|, 22, 23, and 24 of the mainsnetwork D are capacity-coupled by con- 7 densers 25, 26, 21, and 28 tothe transmitters. It

will be seen that the condensers 25, 26, 21, and 28 supplement theseries condenser l and in some circumstances, depending upon thecapacity in that leg of the band-pass filter, the condenser [0 may beomitted. The feeder cables l8 may take the form of single core cablesand in this case the condensers 25, 26, 21, and 28 are connected betweenthe feeder cables and the mains network, at the opposite end of thecable from that shown in the drawing.

It will be understood that various modifications may be made within thescope of the invention. Thus, for example, although the band-pass filterhas been shown connected in the secondary circuit of the outputtransformer of each transmitter, it may be connected in the primarycircuit of that transformer. Again, it may be included partly in thesecondary circuit and partly in the primary circuit. Actually, insteadof a transformer, any other form of impedance transforming network maybe employed. Again, instead of the particular form of filter shown, anarrangement may be used including a series condenser, the capacity ofwhich, together with the inductive reactance due to leakage inductanceof the transformer and the conducting leads, forms a series tunedcircuit which behaves at the resonant frequency as a pure resistancesothat maximum power passes to the mains network D. The same effect canbe obtained by adding additional inductance and reducing the size of thecondenser to correspond. This arrangement, however, forms a tunedcircuit which does not By replacing the condenser by two condensers inseries having the same effective capacity and of such relativemagnitudes that there is a minimum potential to ground at the junctionof the two condensers, it is possible to connect between this junctionand ground, a condenser or an inductance or both a condenser and aninductance either in parallel or in series without disturbing thedistribution of energy at the carrier wave frequency. In this way, aneffective nodal point can be obtained by providing equal condensers andtwo unequal added inductances between them.

In Figure 2, a preferred form of rejector circuit is shown. 1 Thisrejector circuit is in the form of a band-pass filter designed to ensurethat carrier and all side-band frequencies are rejected.

Rejectors in the form of single tuned circuits as shown at 12 [1 inFigure I, prevent absorption of power from one transmitter by another,and also prevent inter-modulation by the carrier and relatively lowside-band frequencies, but it is, of course, desirable that the rejectorshould be effective over a band of frequencies, and the form of rejectorshown in Figure 2 is therefore preferred to be used in the place of thesingletuned rejector circuits l2 ll. Rejector cir cuits of the formshown in Figure 2 present a high impedance to a band of frequencies andtheir reactance at the frequencies it is desired to pass can be absorbedin the band-pass filters of the particular transmitters with which theyare associated. It will be understood that instead of the particularform of rejector circuit shown in Figure 2 any other form of band-passfilter having similar image impedance characteristics may be used.

We claim:-

1. A multi-channel high frequency wire-broadcasting system comprising incombination, a distributing network, a plurality of amplifiers, eacheffective over a band of frequencies and each associated with one of thechannels, a plurality of impedance-matching networks each interposedbetween the output of one of said amplifiers and said distributingnetwork, a plurality of bandpass filters each interposed between one ofsaid amplifiers and said distributing network and a plurality ofrejector circuits interposed between one of said amplifiers and saiddistributing network, each designed to present a high impedance to aband of frequencies, the effective reactance of each of said rejectorcircuits constituting part of the reactance of the correspondingband-pass filter.

2. A multi-channel high frequency wire-broadcasting system comprising incombination, a distributing network, a plurality of amplifiers eacheffective over a band of frequencies and each associated with one of thechannels, a plurality of step-down transformers each interposed betweenthe output of one of said amplifiers and said distributing network, aplurality of band-pass filters each interposed between one of saidamplifiers and said distributing network and a plurality of rejectorcircuits interposed between one of said amplifiers and said distributingnetwork, each designed to present a high impedance to a band offrequencies, the effective reactance of each of said rejector circuitsconstituting part of the reactance of the corresponding band-passfilter.

3. A multi-channel high frequency wire-broad casting system comprisingin combination a distributing network, a plurality of amplifiers eacheffective over a band of frequencies and each associated with one of thechannels, a plurality of step-down transformers each interposed betweenthe output of one of said amplifiers and said distributing network, aplurality of band-pass filters each interposed between one of saidamplifiers and said distributing network and a plurality of rejectorcircuits interposed between one of said amplifiers and said distributingnetwork, and each designed to present a high impedance to a band offrequencies, said band-pass filters comprising pairs of series tunedcircuits, the components of each pair of said series tuned circuitsbeing selected so that the point of connection between them is a minimumpoint of potential.

4. A multi-channel high frequency wire-broadcasting system comprising incombination, a distributing network, a plurality of amplifiers eacheffective over a band of frequencies and each associated with one of thechannels, a plurality of step-down transformers each interposed betweenthe output of one of said amplifiers and said distributing network, aplurality of band-pass filters each interposed between one of saidamplifiers and said distributing network and a plurality of rejectorcircuits interposed between one of said amplifiers and said distributingnetwork, and each designed to present a high impedance to a band offrequencies, the effective reactance of each of said rejector circuitsconstituting part of the reactance of the corresponding band-passfilter, said band pass filters comprising pairs of series tunedcircuits, the components of each pair of said series tuned circuitsbeing selected so that the point of connection between them is a minimumpoint of potential.

5. A multi-channel high frequency wire-broadcasting system comprising incombination a distributing network, a plurality of amplifiers eacheffective over a band of frequencies and each associated with one of thechannels, a plurality of step-down transformers each interposed betweenthe output of one of said amplifiers and said distributing network, aplurality of band-pass filters each interposed between one of saidamplifiers and said distributing network and a plurality of rejectorcircuits interposed between one of said amplifiers and said distributingnetwork, and each designed to present a high impedance to a band offrequencies, the effective reactance of each of said rejector circuitsconstituting part of the reactance of the corresponding band-passfilter, said channels having carrier frequencies chosen near to thecut-01f frequencies of said band-pass filters.

6. A multi-channel high frequency wire-broadcasting system comprising incombination a distributing network, a plurality of amplifiers eacheffective over a band of frequencies and each associated with one of thechannels, a plurality of step-down transformers each interposed betweenthe output of one of said amplifiers and said distributing network, aplurality of band-pass filters each interposed between one of saidamplifiers and said distributing network, a plurality of rejectorcircuits interposed between one of said amplifiers and said distributingnetwork, each designed to present a high impedance to a band offrequencies, a plurality of low-pass filters each interposed between theoutput of one of said amplifiers and said distributing network anddesigned to prevent harmonic frequencies generated in said amplifierfrom passing to said distributing network, the effective reactance ofeach of said rejector circuits constituting part of the reactance of thecorresponding band-pass filter.

7. A multi-channel high frequency wire-broadcasting system comprising incombination, a distributing network, a plurality of amplifiers eacheffective over a band of frequencies and each associated with one of thechannels, a plurality of step-down transformers each interposed betweenthe output of one of said amplifiers and said distributing network, aplurality of bandpass filters each interposed between one of saidamplifiers and said distributing network, and a plurality of rejectorcircuits interposed between one of said amplifiers and said distributingnetwork, and each designed to present a high impedance to a band offrequencies, the effective reactance of each of said rejector circuitsconstituting part of the rea/ctance of the corresponding band-passfilter, said rejector circuits each consisting of reactively-coupledparallel-tuned circuits. v

8. A multi-channel high frequency wire-broadcasting system comprising incombination a distributing network, a plurality of amplifiers eacheffective over a band of frequencies and each associated with one of thechannels, a plurality of impedance matching networks each interposedbetween the output of one of said amplifiers and said distributingnetwork, a plurality of bandpass filters each interposed between one ofsaid amplifiers and said distributing network and. a plurality ofrejector circuits interposed between one of said amplifiers and saiddistributing network, and each designed to present a high impedance to aband of frequencies, said band-pass filters comprising pairs of seriestuned circuits, the components of each pair of said series tunedcircuits being selected so that the point Olf connection between them isa minimum point of potential.

9. A multi-channel high frequency wire-broad.-

casting system comprising in combination, a distributing network, aplurality of amplifiers each eflective over a band of frequencies andeach associated with one of the channels, a plurality of impedancematching networks each interposed between the output of one of saidamplifiers and said distributing network, a plurality of bandpassfilters each interposed between one of said amplifiers and saiddistributing network and a plurality of rejector circuits interposedbetween one of said amplifiers and said distributing network, eachdesigned to present a high impedance to a band of frequencies, theeffective reactance of each of said rejector circuits constituting partof the reactan-ce of the corresponding band-pass filter, said band-passfilters comprising pairs of series tuned circuits, the components ofeach pair of said series tuned circuits being selected so that the pointof connection between them is a minimum point of potential.

PETER PENDLE'ION ECKERSLEY. RUPERT EVAN HOWARD CARPENTER.

